A conventional suction cleaner is a cleaner where part of the cleaning process is sucking up of air, normally mixed with dust, debris, or a dirty water solution, where the dust, the debris or the dirt is filtered off in the cleaner and conveyed to a bag or container before the air is exhausted.
An example of such a suction cleaner is a normal household vacuum cleaner. Other suction cleaners may be specially designed to pick up dirty liquids, examples are e.g. scrubber dryers, carpet extractors, and wet & dry vacuum cleaners. A scrubber dryer is a cleaning appliance which scrubs a surface and recovers the solution, leaving the surface dry and clean. The cleaning is done by applying a solution (water and detergent) to the surface, such as a floor, with a rotating or vibrating brush or a pad. After having worked on the floor for some time, the dirty solution is picked up by an airflow inside a nozzle (a squeegee). Subsequently the dirty solution is separated from the airflow inside the recovery tank of the scrubber dryer.
The airflow is generated by a vacuum motor, which is a unit consisting of a radial fan and an electric motor. Besides the fan for generating the vacuum, the motor may further comprise a separate fan to cool the motor. In order to create sufficient airflow, the unit runs at a high revolution speed, typically 18,000 RPM, for motors running on low voltage from a battery and, typically 30,000 RPM for motors running on voltage from the mains (110 or 230 Volts). This high revolution speed leads to two different sources of noise. A first source is due to the imbalance of the unit, which may never be balanced 100%. The imbalance induces a mechanical vibration with a frequency of typically 300 Hz for low voltage driven appliances and even higher (500 Hz) for mains driven. The mechanical vibration of the vacuum motor—if not controlled—sets the whole body of the appliance into vibration, which may lead to the radiation of a powerful noise from the body with a frequency of typically 300 Hz-500 Hz. Another source of noise is aerodynamic in nature and is due to the high air velocities inside the main fan and the cooling fan of the vacuum motor. The aerodynamic source may again be divided into two different types of noise; a broadband noise over a wide frequency range (typically 500 Hz to 10,000 Hz) arising from the turbulent nature of the flow, and a narrow band noise (with a number of discrete frequencies only typically in the range 2500 Hz to 6000 Hz). The narrow band noise is due to the fact that the fans have a finite number of fan blades.
Normally, the noise from the exhaust of air from the main fan is reduced by an absorbing material such as rubber foam. In order for the foam to be effective it must be placed close to the exhaust air, and in order to obtain this the wall of the exhaust duct is covered by sound absorbing foam. However, the use of foam may cause two problems: the first problem is that the foam may easily block the air path of the exhaust air and, consequently, reduce the airflow and the recovery performance of the appliance. The second problem is the ability of the foam to suck up moisture, which may lead to growth of mould and bacteria, thus, making the appliance unsanitary. Foam is also an extra part which adds costs to the appliance. The noise from the fan cooling the motor may be controlled in a similar way; however, in many suction cleaners this noise is not controlled at all.
The noise from imbalance is normally controlled by a flexible mounting such as a rubber gasket between the vacuum motor and the scrubber dryer body; however, since the vacuum motor is often bolted to the body, the effect of the flexible mounting is often rather limited.
In recent years, the technique of a reactive silencer has been introduced on cleaning appliances with vacuum motors. The principle of a reactive silencer is to change the acoustic impedance of the exhaust duct to minimize noise propagation rather than absorbing the noise energy and to transform the noise energy into heat. This noise reflection with a reactive silencer occurs where the exhaust duct changes cross-sectional area or direction. One well known principle is to release the exhaust air and noise in a relatively large cavity and then connect the exhaust duct to this cavity. The noise related to the release of exhaust air from the fan may then be controlled by the volume of the cavity, and the length and width of the exhaust duct. The best result is obtained for a large cavity, and a narrow and long exhaust duct. This principle is explained in the European patent EP 1 266 501 B1, which also discloses how to obtain a long and narrow exhaust duct, e.g. by the use of a serpentine shaped duct. A similar principle of noise reduction in a vacuum cleaner is known from the Chinese patent CN100556352 C.
However, the known techniques mainly aim at reducing the noise related to the fan for generating the main airflow used for pick-up.